Displayed or printed color images obtained from scenes captured underwater are typically very disappointing to the amateur photographer. When an image is captured underwater, the red portion of the illuminating light is largely filtered out by the water. This filtering effect is more pronounced at greater depths and at increased distance from the subject to the camera and results in images lacking the appropriate amount of red illumination. Images captured underwater thus appear shifted to the blue or green portions of the spectrum. Additionally, contrast and sharpness of such images are often disappointing.
The captured image may consist of an accurate rendition of the scene content; however, it is typically not what the photographer or diver remembers of the scene. Most photographers automatically interpret the scene to be much more vibrant, likely due to a complex interaction of the human visual system and the brain. What photographers remember is a much higher contrast image with brighter colors.
In the underwater environment, available light for photography is reduced and photography becomes more challenging with greater depth, as the red illumination is filtered and the overall illumination decreases. This problem persists even when underwater illumination systems are provided, because the red illumination is filtered by the amount of water between the underwater illumination source and the subject; this filtering effect increases due to the amount of water in the return path from the subject to the camera. As a result, underwater images often suffer from poor image quality when reproduced by traditional systems.
Fredlund, in U.S. Pat. No. 6,263,792, describes a method for printing images captured underwater. Image analysis is performed to determine if images were captured underwater. Images captured underwater are then subjected to a predetermined correction. Amplification of the red illumination channel is described. Additionally, averaging out high frequency noise in the amplified red channel and contrast expansion in all the channels is taught.
Placing a filter, such as one which passes red wavelengths and attenuates blue and green, over the camera's lens and metering system can help to improve image quality; however, this filtering decreases the overall light level and maintains a fixed ratio between the color channels. A fixed ratio can yield an undesirable amount of correction because the relative illumination in the red channel decreases with respect to blue and green with increasing depth and distance to the subject.
In U.S. Pat. No. 5,719,715, Westhaven describes a family of filters for use in capturing images that attenuates blue and green light while passing red. However, use of such filters requires changing to the appropriate filter for each depth or distance to subject. This procedure can be tedious and error prone, poorly suited for the amateur underwater photographer.
While flash systems and other artificial lighting can be utilized, the distance between the subject and camera must also be considered. While macro exposures are often used to minimize this distance problem, close-up photography may not be practical or desirable with many subjects. Additionally, use of flash systems often introduces other problems.
Video capture provides another set of problems. Generally, illumination systems are not employed for video use, and the exposure time is limited by the video frame rate. The red channel is affected in the same manner as with still image capture, and overall sharpness and contrast are also reduced.
Post capture image modification can also be used to correct these problems for both still and video capture, but the techniques used require expertise in image manipulation, utilizing expensive and complex software tools and are often very tedious to use. Additionally, the same image improvements applied to one underwater image will not necessarily improve the appearance of another image captured underwater. There is a great deal of variance in images captured underwater with the same equipment, due to variations in depth, distance to subject, water quality, available sunlight, flash illumination, and local water color. Hence, this type of modification must be tailored specifically to each image, increasing the time requirement and the need for expertise when improving a number of images.
There exists a need to improve the quality of images captured underwater without employing cumbersome and expensive equipment and without requiring use of software tools requiring expertise in image manipulation techniques.